A micromechanical electronic system (MEMS) device may have a cavity in a substrate underlapping an overlying layer. The cavity may be formed using an isotropic etch process through an access hole in the overlying layer. Forming the cavity with lateral dimensions within desired tolerances may be problematic due to interaction of several etching parameters, such as pressure, temperature, concentrations of reactant species, and geometric factors of the cavity with respect to access of the etchants.